The growth and arborization of chicken retinal ganglion cell axons have been investigated by means of an intraaxonally transported fluorescent marker in the developing retinotectal system. The fluorescent dye D282 or diI from the carbocyanine group of dyes is taken up by ganglion cells and labels the axon as well as the axonal growth cones and the terminal arborizations on the tectum. Branching and arborization start in the chick retinotectal system on embryonic day 9 (E9). At this stage retinal axons leave the stratum opticum (SO) and invade the stratum griseum et fibrosum superficiale (SGFS), where arborization takes place. On day E12 several axons were found to arborize in the SGFS. At this stage arbors appear to have small branches with less than 4 branching points. The extension of terminal arbors in the anterior/posterior (A/P) and in the dorsal/ventral (D/V) direction was determined for 50 axonal trees at days E13-14 and for 24 arbors at days E15-16. Few axonal terminals were investigated at day E18. The mean A/P extent of axonal terminal trees increases from 0.23 +/- 0.12 to 0.36 +/- 0.22 mm from E13-14 to E15-16 and seems to stay at this order of magnitude on E18. The mean D/V extent increases from 0.23 +/- 0.17 to 0.30 +/- 0.18 mm in the same embryonic period of development. The number of branching points calculated from the same number of axonal trees increases from 7.50 +/- 2.98 at E13-14 to 11.70 +/- 4.10 at E15-16. This number seems to increase further after day E16 achieving values of about 20 to 25 at E18. This was, however, not quantifiable by the technique used here and represents an approximate value estimated from 6 completely labeled terminal fields at E18. The data presented here suggest that the modeling of the final branching pattern in the chick retinotectal system takes place within a relatively short period of embryonic development. Prior to the beginning of terminal arborization two important events contribute to the formation of a retinotopic projection. One event is the change of the D/V position by a minority of axons lying ectopic in terms of retinotopy. Some axons turn at right angles and change their D/V position. The other event is the appearance of side branches along the A/P axis.